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I'd like to use a vacuum gauge to test older, high mileage vehicles I'm thinking of purchasing. I figure I'll probably be limited to this, checking with a multi-meter, a balance test and whatever I can see with my eyes.

I've found various sites, for example here, here and here, which detail the various results you can get and what they indicate is wrong.

Most seem to agree that normal is between 17-22 at sea level subtracting 1 for each 1000 feet of elevation.

What's not always clear is which conditions are serious, walk away from this car problems, and which ones might be things that could be cheaply fixed either in a shop or as a small DIY job.

For example, a steady low reading between 8-16 could be attributed to any of the following:

  • Incorrect valve timing
  • Incorrect ignition timing
  • Small vacuum leak
  • low compression/worn rings

Will the quick open/close test tell me for sure that it's worn rings? If it is, what's the walk away threshold? 14? 12? 8?

While a compression leak between cylinders or a leaky head gasket seem like walk away problems, what about other things listed like:

  • Sticky Valves
  • Weak valve springs
  • Worn intake valve stem guides
  • Leaky manifold or carburetor gasket
  • Excessive exhaust back pressure (plugged muffler or catalytic converter)

I just want to get as much info out of this test as I can.

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  • Using a vacuum gauge would be very iffy. You don't know what has been done to the vehicle. Generally, a vehicle with a "large" cam, for instance, would have a low vacuum reading. Also, a vacuum reading would only give you the average of all cylinders. A better way to test a vehicle is by doing a compression test. This will give you the health of each cylinder. Plus, you can research what a good reading for the engine you are looking at should be. Jun 20 '15 at 20:49
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    @Paulster2 I don't think the locals here would put up with me starting to do more invasive tests like a compression test. Jun 20 '15 at 21:06
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    I hear you on that. You can always ask them to do one for you. Best I got for you, brother. Or, take it out for a long <cough> test drive ... Jun 20 '15 at 23:35
  • I've always been more in favor of a leak down test instead of a compression test for evaluating an engine. The engine does not have to run either, so the test can be run on junk yard motors before installation.
    – vini_i
    Jan 9 '16 at 12:19
  • I know that I no way answered your question with my response but I think the vacuum test couple with my response gives you really good info. perhaps you do the vacuum test and if the results leave you with not enough data to determine a course of action you follow up with what I answered with. Jan 15 '16 at 23:31
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I use a leakdown tester

If I'm buying a used car or motorcycle I use either a leakdown tester or simply compressed air.

I have an air cannister with an air regulator, so I can control the flow of hte compressed air, that I can fill with compressed air, it's a finite quantity so I have to be sure to get it right the first time or a run out.

I also bring a bump starter to get the motor at TDC easily and a car stethoscope to use as a listening device.

Compressed Air Test

  • If you don't have a leakdown tester you can use this method. You will need to have the various couplings to screw the air hose into the sparkplug holes. Remove the spark plugs, obviously. Use the bump starter to get the piston to TDC on the bore you want to test. Insert your air adapter into the spark plug hole and let air out of the canister using the regulator. Be easy, you don't want the air pressure to force the piston down.

  • Rings - Listen for air leaks into the crankcase. You can listen at the oil filler cap, not so good, or insert the stethoscope into the oil dipstick tube, better. If you hear a lot of air moving into the crankcase you can determine if the rings are good or not.

  • Exhaust Valves - Listen in the exhaust pipe. Cat converters make this harder as they will absorb sound wave energy. This isn't as effective on cars as it is on motorcycles. You can also hold the stethoscope against the exhaust pipe after the more solid manifold and hear air escaping the combustion chamber from a bad valve.

  • Intake Valves - Same as above but listen at the throttle body or carb.

Leakdown Tester

You can follow this generic test. The procedure will vary slightly depending on the leakdown tester you own. The description in the link is very good though.

Recommendation

Do the tests a few times at your home to get your procedure nailed down. You don't want to run out of compressed air or you have to go and recharge it and come back. You only run out if you do the test and you have some valves open because your not at TDC. I've never had it happen to me but I'm always worried about it when I'm doing it.

I believe these tests coupled with the test you have described in your question above would sniff out the answer to your question of when you should walk away or not.

Bump Starter

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Compressed Air Canister

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Stethoscope

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